Scatter Particles after the have been created
Add an appropriate force field, or maybe fields if you want the "centers" of the explosion to be on each individual letter.  Keyframe the strength of the force field to be zero on frame 150, then something higher (say, 10) on frame 151.  Set the force of gravity of the particle system to zero (or zero until frame 151 if you want your explosion to include gravity effects).

Here I have keyframed the explosion to start on frame 101.  So on frame 100 the strength of the field is zero and the particles have not moved:



A few frames later, they have exploded outward: